1. Field of the Invention
The present invention relates to a coding technology and a decoding technology of a video signal.
2. Description of the Related Art
In recent years, a service to distribute digitized image and audio contents via a broadcast wave such as a satellite and a ground wave, and a network has been realized, and a highly-efficient coding technology for efficiently recording and transmitting contents having a massive amount of information is being required. Used as the highly-efficient video coding is a method for compressing information using a correlation between spatially adjacent pixels in the same frame of a video signal, and correlations between temporally adjacent frames and fields, as represented by MPEG-4 AVC.
In MPEG-4 AVC, used as compression using a temporal correlation is motion compensated prediction to use a locally decoded image of an already-coded frame as a reference image for a target image being a coding target frame, detect the amount of motion (hereinafter described as “motion vector”) between the target image and the reference image in units of two-dimensional blocks (hereinafter described as “target blocks”) of a predetermined size, and create a prediction image based on the target block and the motion vector.
In MPEG-4 AVC, it is possible to improve the prediction accuracy of motion compensated prediction by using a prediction method using a motion vector of each target block by making the size of a target block variable in a two-dimensional block of 16×16 pixels being the unit of an encoding process (hereinafter described as “macroblock”), a method for storing a plurality of reference images to select a reference image to be used for prediction, and a method for obtaining motion vectors between two reference images and a target block to create a motion estimated image, and accordingly a reduction in the amount of information is realized.
Moreover, it is necessary in motion compensated prediction to encode and transmit a created motion vector. In order to prevent an increase in the amount of information due to motion vectors, a predicted motion vector value that is predicted from motion vectors for already-decoded blocks around the target block is used for coding; accordingly, it is possible to use motion compensated prediction called a direct mode that does not transmit a motion vector.
However, since the above prediction of motion vectors cannot necessarily be obtained with high accuracy, a method for detecting a motion vector between reference images on both of the coding and decoding sides, assuming that the motion vector is temporally continuous, creating the predicted motion vector of a target block, and configuring the direct mode is also presented as shown in Patent Document 1.